The ON-OFF modulation of the power electron beam (in the order of 1 kW or tens of kW) emitted by an electron gun has hitherto been obtained in various ways. One of these is the use of a control electrode (grid) to which there is alternately applied a negative potential in the order of tens/hundreds of volts with respect to that of the cathode. The potential is applied in a manner as to obtain beam prohibition (OFF), and at a potential close to that of the cathode or even positive, to permit beam emission (ON). Such a solution involves the generation of a dedicated voltage, modulated between two levels, and its application to an appropriate electrode (grid) (FIG. 1).
Another problem with regard to grids is their resistance to mechanical and thermal stresses and thus their reliability, which is important especially in the space and occupational field.
Further solutions, which do not take into account the use of a dedicated electrode (grid), modulate the accelerating voltage applied to the electron gun at the desired frequency or act on the cathode emission, the latter however being band-limited to a few Hz by the thermal time constant of the cathode.
Systems already used in the space field are:
the interposition of a solid state interrupter in series with the cathode bias (FIG. 2), which has permitted the achievement of the ON-OFF modulation of a beam using 1 keV, 100 mA [P. Banks et at., VCAP EXPERIMENTS ON STS-3], i.e. a beam of limited power; this is because such a solution implies the interruption, at high tension, of the cathode emission current; PA1 the use of high-tension power generator, itself having a modulated output, as generator of the accelerating voltage. In this case, it is necessary to provide: ON-OFF modulation of the full output power; optimization of the design of the generator having regard to the load (characteristics of the gun).